Method of increasing the fatigue life of rolling contact elements and the resulting articles



N 1969 H. s. SCHLICHT 3,477,884

' METHOD OF INCREASING THE FATIGUE LIFE OF ROLLING CONTACT ELEMENTS AND THE RESULTING ARTICLES Filed July 1'7, 1967 2 Sheets-Sheet Nov. 11. 1969 H. sfscr-lucm 3,477,884

METHOD OF mcnmsmc THBFA'IIGUE LIFE OF ROLLING CONTACT ELEMENTS AND THE RESULTING ARTICLES Filed July 17. 1967 2 Sheets-Sheet 2 Sch . Int. c1. F16c 33/12, c21e1/42, 1/10 s. or. "148- 150 8 Claims 1 .ABSTRACT on THE DIsc osuRE Theus'eful fatigue life of rolling contact elements (such =asantifriction' bearing elements,'gears androllers in sheet metal rolling mills) is increased by imparting to at least "one of the rolling elements, in the region thereof that is subjected to the main shearing stress'under load, a residual compressive stress acting in opposition to and counteracting, said main shearing stress. This isaccomplished by a combination of induction heating to selectively *austenitizea' surface zone adjacent the rolling contact "surface,"quenching to cause a selectively'higher dilatation in the s'urfac'e zone, and induction tempering of the quenched element so as to produce in the surface zone a =vblu'mecontraction which ditfers from the dilatation of the underlying zones. Theresult is to impart anopposing United States Patent 0 residual *compressive stress which initially'increases with increasing distance inwardly from the surface, reaching a maximum approximately at the depth where maximum shearing stress is expected to occur and'thereafter-decreasing. t t

-,-..This application is a continuation-im y prior .n-ficopendin' applicationSer. No. 390,011, fil d 17, 964,nowabandoned.

*This invention relatestoa method of increasing the jfatigue life ofrollingcontact elements the parts of which It 'is'known' that the fatigue life of a material can be increased=by imposing astate of residualstress on which "is oppositeto the critical stresses" occurring with its loads. "The lowtensilestrengthof-concrete can for example be *increased'by prestressing it inthe direction'of the pressure.

have already-"as his already known--been mechanically treated byfor'exampl'e hammering, rolling, sandblasting "or grinding in order to obtain a state of residual stress on the surface'which,however, has only a low penctration depth (only a'few ,u). Each'of thesemechariical treatments is, aside from the requiredypreviously 'performed heattre'atment, an additional process by whichthe price ofthe product isappreciably increased'.

' Realizing these disadvantages, many attempts have been made heretofore to perform the quenching process, which the hardening material has beenexposed to air forfsome time before quenching or'it has'been immersion-quenched. With all these treatments, however, 'it has not been possible heretofore'to increase the fatigue life of hardduring the austenlte-martensite transformation in the surened steel workpieces, as represented by rolling contact 3,417.7,884 Patented Nov. 11, 1969 ice elements, in a satisfactory way. Therefore, as it is already known, the attempt has been made to influence the state of residual stress of workpieces of hardening steel by aditferentiated austenitizing of the surface zone and the interior of the part. For this reason, either with an unchanged composition throughout the cross section of the steel different thermal treatments are performed between the surface zone andthe interior or, with a changed composition throughout the cross section of the steel, a similar thermal treatment for hardening is performed between the surface zone and interior. On the one hand, the surface hardening treatments, as they are used for flame hardening-or induction hardening, can be mentioned as examples; On the other hand, methodssuch as carbur'izmg (case hardening) or nitro-carburizin'g, come into "question, which result in a carbon or carbon-and nitrogen content decreasing from the surface zone to the interior.

With all these hardening methods compressive stresses, which decrease from the surfacezone towards the interior of a steel workpiece, can be produced).

So far as an increase of the fatigue life of rolling contact elements is concerned, it is already well-known that their state of residual stress can be influenced by differentiated austenitizing of the surface zone and the interior.

time being is transformed to martensite and the surface zone is plastically deformed.

With the subsequent transformation of the surface zone the already martensitic interior cannot be plastically deformed any more, which results in compressive stresses at the surface zone.

A second variant is that another element influencing the Ms-point, preferably carbon or nitrogen but also nickel, chromium, or boron, is diffused into the surface of the steel workpiece during austenitizing whereby the Ms-temperature is lowered in the above described way. 'Finally, a third variant results from the fact that, for producing a greater concentration of the element influencing the Ms-point, a heat treatment is performed to begin with and thereby carbon or nitrogen (or nickel,

chromium, or boron) will be diffused, and subsequently austenitized.

. The above mentioned three variants are, however, only able to produce residual stresses the maximum of which lies immediately at or in the surface zone of the workpiece. Moreover, the depth effect of the imposed compressive stresses is low.

The present invention, on the other hand, concerns a method of increasing the fatigue life of rolling contact elements, this method comprising the features that at least oneof the elements in the zone of its maximum shearing-stress, which occurs during rolling under load, is, provided with a counteracting residual stress, and in which particularly this element, consisting of hypereutectoid ferromagnetic steel, is austenitized in a surface zone close to the raceway by means of inductive heating more uniformly and with a higher degree than the underlying zones, utilizing the known skin-effect, at first, for heating the surface zone up to a temperature threshold,

then for increasing the temperature to above the Curie point whereby heating is carried on over the threshold to the interior, whereupon the element is quenched, so that face zone it, together with the carbon which is more heavily incorporated in it, is increased in volume to a higher degree than the underlying zones. This dilatation, which is different between the surface zone and the interior, produces a nearly uniform residual compressive stress within the surface zone, and considerably lower pressure in the underlying ZOneS, so that an inductive tempering of the quenched element, which according to the present invention is again performed utilizing the skineffect, lowers the nearly uniform residual compressive stress of the surface zone in the immediate ambiency or vicinity of the rolling surface.

In this manner, the element treated in the described way is given a residual compressive stress which in the beginning increases from the rolling surface, which then reaches a maximum with increasing distance from the rolling surface (according to the depth of the maximum shearing stress) and which then drops in the direction of the interior. The depth of the compressive stress maximum to be produced according to the depth of the shearing stress maximum is essentially dependent on the geometrical shape of the element and on the load. As the specific load of the above mentioned elements is normally between 250 and 350 kg./mm. the value for the depth of the stress maximum with normal ball or roller bearings is about percent of the smallest radius of curvature of the paired rolling contact elements.

Another feature of the method according to the present invention is that a rolling contact element during quenching is immersed into a salt bath of 160 to 170 C.

An additional feature of the present invention is the fact that the above mentioned methods can be used for gears or for rollers in sheet metal rolling mills.

These and other features and advantages of the present invention are describedhereafter in detail and are explained by means of the accompanying drawings, wherein:

FIG. 1 is a diagram which shows the stresses of a rolling contact element occurring under load and the residual stress of the element, neutralizing those stresses;

FIG. 2 is a diagram which shows the changes in volume of a rolling contact element that take place in dependence on the quenching temperature;

FIG. 3 shows a curve set illustrating the inductive heating of a rolling contact element for different values of time; and

FIG. 4 shows a curve set illustrating the influence of the inductive tempering on a rolling contact element which has been hardened according to the present invention.

According to FIG. 1, on the surface 1 of a rolling contact element, for example a ball bearing raceway, there rolls another similar rolling element, for example a cylindrical roller the surface of which is marked 2. Because of the load, according to a theory of Heinrich Hertz of the year 1895, a pressure diagram 3 is formed which within the contact area of both elements results in the illustrated flattening of the rolling element 2 on its raceway.

Because of the pressure diagram 3 normal stresses occur in the material below the surface of the rolling element surface 1, the shape of these normal stresses being plotted according to the orthogonal directions of the coordinates by the curve set 4 in the load direction 5, andby the curve set 6 vertically to the load direction 5. The difference between these stress values furnishes, for each elementary or unit cube in the interior of the raceway, shearing stresses the extent of which-apart from an integral factor-proceeds beginning from a point 7 of the main stress below the apex 8 of the pressure distribution 3 in the stress direction 5 as is illustrated by the curve set 9.

According to this, therefore, the fatigue of the material which is developed in its interior by the shearing stresses increase firstly below the raceway surface 1, then reaches a maximum and, finally, drops in the direction of the interior of the workpiece. This theoretically determined development is later on confirmed by the so-called pittings, which occur during rolling contact on account of fatigue of the material, and always just below its surface, exactly in the zone of the maximum shearing stress.

According to the previously outlined purpose of the present invention, the rolling element limited raceway 1 is, by the method of the present invention, provided with such a prestress as is illustrated by the curve set 10. This prestress is the mirror image of the curve set 9 which is formed by the rolling element 2 below the raceway 1. Particularly, both curve sets 9 and 10 show maximum values 11 and 12 of the ordinates at a greater depth the raceway surface. Under normal operating conditions this depth is, as mentioned above, about 5 percent of the smallest radius of curvature of paired normal rolling contact elements. This radius, thus, is between and mm. Accordingly, the position of the maxima 11 and 12 is, with respect to the dimension of the radius of curvature which here has been taken as the base, considerably distant from raceway 1.

In order to produce a residual compressive stress in a rolling element corresponding to the curve set 10 in FIG. 1 which is largely the same (symmetrically) as the stress relationship alOng line 9 occurring under rolling contact of a rolling contact element, a surface zone close to the surface of a rolling contact element (1 in FIG. 1) is-as has been indicated beforeaustenitized, prior to quenching, by inductive heating more highly and more uniformly than the underlying zones. In that manner, at the transition point from the surface zone to the underlying zone a threshold of different states of austenitizing is formed. By quenching the rolling element a transformation of austenite to martensite and, thereby, an increase in volume is effected. This is the greater the more highly the material has been austenitized. Accordingly, the more highly and more uniformly austenitized surface zone increases more in volume than does the underlying zone. These different increases in volume result in a state of compressive stress wherein the stress, with increasing distance from the surface of the rolling element, remains nearly the same up to the depth of the threshold formed during the previously mentioned austenitizing treatment, and then drops thereafter. The position of the threshold is about the distance of the compressive stress maximum (12 in FIG. 1), which is developed by further (yet to be discussed) treatment of the workpiece, to the surface of the workpiece.

The metallurgical processes occurring in the rolling contact element prior to quenching are based on the fact that annealed hardening-specially hypereutectoid-steel consists of iron and carbon, whereby iron occurs in the form of ferrite in body-centered cubic crystalline structure and carbon is combined with iron to form the carbide. The solubility of carbon in the ferrite matrix is low. The steel being heated to hardening temperature (about 850 C.), the body-centered cubic ferrite matrix is transformed to the face-centered cubic austenite matrix. Simultaneously, carbide begins to dissolve, giving off carbon, whereby carbon and iron form a solid solution.

I The above described processes are dependent upon temperature and time. The higher the temperature and the longer the period of time, the more carbide is dissolved and the more carbon will be incorporated in the austenite matrix. With slow cooling from hardening temperature these processes are effected in reverse order. Then, by diffusion, carbon is separated again and carbide is formed anew. When, however, the steel is rapidly quenched from the hardening temperature, the carbon remains incorporated in the matrix. Falling below the socalled Ms-temperature, austenite transforms to martensite, developing a hard and brittle matrix. With transformation of austenite to martensite the above mentioned increase in volume is obtained. It is the greater the more carbon has been incorporated in the austenite, namely up to 4 percent with hypereutectoidi.e., those steels the carbon content of which is more than 0.79 percent. During the austenitizing the carbides dissolve and the carbon is the more incorporated the higher the austenitizing temperature and/or the more time has been allowed for austenitizing. With the transformation from austenite to martensite the dilatation or expansion that occurs is the greater the more carbon has been incorporated in the austenite.

. The transformation from the austenite matrix to martensitic structure, which is effected by quenching, is performed, as is shown on the diagram of FIG. 2 by the curve set 13, passing in direction of the arrow 14, in which are plotted the specific dimensional changes, dl, occurring with quenching as a function of the temperature T. The point of the surface zone of the workpiece, which most intensely comes into contact with the employed quenching medium (for example, oil) experiences firstly a cooling up to the temperature Ms. The herewithoccurring transformation of the matrix from austenite to r'nartensite structure, and thereby from face-centered cubic matrix to body-centered cubic matrix, involves a dilatation of volume which is effected from value dl to value dl on the dl-axis of the diagram. The interior experiences-temporarily deferred, but with the same temperature-the same dilatation in volume.

With case-hardening it is different. The transformation of the interior with a considerably lower carbon content, from austenitic to martensitic structure, begins with the elevated temperature Ms and, compared with the surface zone, isefiected with a substantially lower dilatation of volume as is indicated by the distance of the values dl and dL; in FIG. 2. Because of the different dilatation of the surface zone and the interior a residual compressive stress is developed within the element, which extends, though constantly decreasing, from its surface, for example, the raceway 1 in FIG. 1, to its interior.

The inductive heat treatment according to the present invention, involves, as well prior to as after, the quenching of a rolling bearing element, compared with the indirectly-acting heat-treating methods as results from convection and radiation, the advantage being that the heat is from the very beginning generated within the element, i.e., in a surface layer of lower depth (skin-effect). This penetration depth of the electric eddy currents causing heating depends, as is known, on the specific resistance of the material, its magnetic permeability and on the frequency of the electric current in the induction coil and, consequently, is readily controllable by a suitable selection of the last mentioned independent variable. It is thus possible to produce the shape of the curve set in FIG. 1, and thereby the compressive prestressing imposed on the rolling element according to curve set 9, i.e. accordingto the shearing stresses developed by the load, and thus effectively to neutralize these harmful shearing stresses.

Upon inductively heating a rolling bearing element consisting of hypereutectoid ferromagnetic steel, not only its surface subjected to the skin-effect is heated. Because of the heat conductivity of the material the heat even penetrate-s beyond the threshold Sch in the diagram of FIG. 3 in the direction of the interior K. The advance of the penetration temperature T can be seen from the curve set corresponding to the time parameter t According to this, the temperature drop up to the penetration depth of the eddy currents is relatively slow; it then continues, however, to drop abruptly in the direction of the interior K.

The inductive heating of rolling contact elements according to the present inventionis effected up to above the Curie-point, whereby the frequency of the alternating current is adapted to the requirements of the process. Exceeding this temperature value the permeability of the matrix changes suddenly by a magnitude of two decimal powers, whereby the electric penetration depth is also changed. In this way, the temperature drop from the raceway surface (FIG. 1) to the surface in the interior of the material, which had been heated to Curie-temperature, is only slow; it then continues, however, to drop abruptly in the direction of the interior K.

This relationship can likewise be clearly seen from the diagram of FIG. 3, in which the temperature advance T in the direction of the interior K of the material is plotted for different time parameters (up to i According to this figure, the surface zone reaching up to the threshold Sch has a temperature advance which guarantees a higher and more uniform austenitizin g.

The utilization of this threshold is of outstanding importance for the present invention. A higher austenitizing corresponds to a greater number of carbon atoms incorporated in the austenite. The quantity of these atoms determines, however, the increase in volume which, accordingly, by a cor-responding selection of voltage and frequency of the induced current and its duration of effectiveness, can not only be changed in extent but also in position within the rolling element, and which can therefore be adapted to its critical loads, so that the stresses developed thereby--as can be seen from the shape of the curves 9 and 10 in FIG. 1be fully neutralized.

According to the features of the present invention, the heat treatment after quenching is also of great significance so far as its purpose, namely the increase of the fatigue life of rolling contact elements, is concerned. Normally, to influence its technological properties hardened steel is tempered, that is subjected to elevated temperatures for a longer period of time. Such a treatment causes again transformations of the matrix which are connected which changes in volume. As an example it can be mentioned that during the tempering of the steel Cr6 SAE 52100) mentioned below at temperatures up to C., because of the tetragonal-cubic transformation of the martensite a contraction takes place, While tempering with temperatures up to 240 C. (because of the transformation of the retained austenite) causes a dilatation and, finally, tempering with temperatures of more than 240 C. (because of the further decomposition of the martensite) causes still another contraction.

Such changes in volume which occur with normal tempering of a hardened bearing steel do not cause any noticeable changes of stress as the changes in volume simultaneously occur outwardly and inwardly of the workpiece. With the presence of a temperature gradient durliiig tempering, corresponding changes in the volume res t.

With inductive tempering employed according to the present invention wherein the heating of the workpiece is eifected again by way of its skin, a temperature gradient occurs. It is controllable by corresponding selections of voltage and frequency of the induced current and its duration of effectiveness. In this way, outside zones of workpieces can be tempered with temperatures above 240 C. while the underlying layers are tempered with lower temperatures. It is also possible to temper only theouter zones with lower temperature. In both cases a contraction of volume of the outer zone, compared with the underlying layers, is effected.

Prior to tempering, a compressive residual stress is present (as is illustrated in FIG. 4 below the raceway surface 1 by the dotted curve 14) which then changes over into the curved section 10' (see FIG. 1). According to this, a nearly uniform pressure prevails from the surface zone to the transition point 15 of the curves 14 and 10, i.e., up to a certain depth below the raceway surface 1, and thereafter this pressure drops in direction of the curve 10' towards the interior K. During tempering this pressure is relieved in the outermost surface zone up to values resulting from curve section 10". The curve section 10 does not begin in zero N on the system of coordinates but, with a certain prestress value W already along the x-axis. Apart from this not unimportant detail, the total shape of the curve resulting from the sections 10' and 10" is such that it is largely adapted or corresponds to curve 10 of FIG. 1 and C Si Mn Cr 1 S 100 Crfi O. 95 O. 15 0. 25 1. 40 Max. Max. 7 1. 05 .0. 35 O. 40 1. 65 O. 025 0. 025 100 C1Mn6 0. 95 0. 50 1. 1. 40 Max. Max. 1. 0. 70 1. 2 1. 65 0. 025 0. 025

100 CrMOG O. 95 O. 20 O. 60 1. 60 Max. Max. 1. O5 0. 40 0. 80 1. 80 0. 025 0. 025

A roller consisting of 100 Cr6 of the above composition and with the dimensions 30 x 48 mm. is to be through hardened: it is induction heated in a coil with the dimensions 40 x 75 mm. wherein it is subjected for 116 seconds with a field intensity of 360 a.w./cm. to an alternating magnetic field of 10 kHz. Quenching is effected in a molten salt bath of 160 to 170 C., with subsequent cooling in air.

By inductive tempering with a field intensity of 60 a.w./cm., employing a frequency of 10 kHz. and a penetration time of 116 seconds, the shape of the stress before tempering, as is illustrated in FIG. 4 by the curves 14, 10', is changed to the final curve shape 10', 10'.

It is, of course, possible to use other quenching mediums as for example, water, brine, oil etc., and, proceeding from the same austenitizing threshold, to fix different states of stress.

It is to be understood that although the present invention has been described in connection with certain specific examples, it is not intended that the invention be limited thereby except as defined in the appended claims.

I claim:

1. In a method for increasing the useful fatigue life of rolling contact elements by providing at least one of the rolling elements in its zone subjected to the main shearing stress under load with a residual compressive stress acting in opposition to the main shearing stress, the improvement comprising induction heating at least one element made of hypereutectoid ferromagnetic steel to austenitize a surface zone adjacent the rolling contact surface of the element more uniformly and to a greater extent than the underlying zones by making use of the known skin-effect and by increasing the temperature of the surface zone above its Curie-point, quenching said element to cause during the austenite-martenzone and considerably less stress in the underlying zones; induction tempering the quenched element by making use of the skin-effect for a second time in .a manner to produce in the surface zone a volume contraction which differs from the dilatation of the underlying zones to relieve the nearly uniform compressive stress in the portion of the surface zone immediately adjacent the rolling contactsurface of "the element, whereby said element is provided With a' residual compressive stress which initially increases with increasing distance in.- wardly from the surface, reaching a maximumapproximately at the depth where maximum shearing stress is expected and thereafter decreases.

2. A method according to claim 1, wherein the element is quenched in a bath of oil or water or brine or molten salt with a temperature between and C. 1

3. A method according to claim 1, wherein the rolling contact elements comprise an antifriction bearing element.

4. A method according to claim 1, wherein the rolling contact elements comprise gearing elements.

5. A method according to claim 1, wherein the rolling contact elements comprise mill rollers.

6. A workpiece having increased fatigue strength and useful life comprising a body element of a throughhardening ferromagntic steel, said body element having a contact rolling surface zone and underlying zones of different residual prestressed characteristics comprising a compressive prestress which increases with increasing inward distance from the surface of the body element, reaching a maximum at the approximate depth at which maximum shearing stress will occur during operation, and thereafter decreases.

7. A workpiece according to claim 6, wherein the body element is a finished rolling contact bearing race element of a thickness up to 25 mm.

8. A workpiece according to claim 6, wherein the approximate depth at which said maximum shearing stress will occur during operation ,is about 5% of the smallest radius of curvature of the paired rolling contact elements.

References Cited UNITED STATES PATENTS 1,783,764 12/1930 Adams 148l50 2,730,472 1/1956 Osborn 148150 3,117,041 1/1964 Koistinen 14839 OTHER REFERENCES Sherman, Megacycle Induction Heating, Steel, March 12, 1945, pp. 116, 156, 158, 160, 162, 164 and 167.

ASM preprint 22, 1 945, Metallurgical Characteristics of Induction Hardened Steel, Poynter, pp. 1-10 and 24 CHARLES N. LOVELL, Primary Examiner U.S. c1. X.R. 148--39,-152; 308-1, 241 

